Frontal-Polymerization-Based 3D Printing of Thermoset Polymers and Composites—Experiments and Modeling

A recent addition to the family of Direct Ink Writing (DIW) methods for 3D printing, Frontal-Polymerization (FP)-based printing has shown promises in the rapid and accurate creation of complex-shape parts made of thermoset polymers and polymer-matrix composites. FP is a process in which a localized reaction zone, driven by the heat generated through an exothermic reaction, propagates through the monomer or gel by converting it to a polymer. By taking advantage of the self-sustained propagation of the polymerization front, the FP-based printing process combines the printing and curing processes by frontally polymerizing the printed filament gel upon extrusion from the printer head. By concurrently solidifying the filament during extrusion, the FP-based process allows for the creation of 3D freestanding objects of complex shapes without the need for a post-processing step. The process can also be adapted to incorporate nano-fillers to produce multifunctional thermoset composites with targeted thermal, mechanical, or electrical properties. The addition of second-phase materials (carbon nanotubes, carbon black particles, short fibers, …) have an impact on the rheological properties (viscosity, extensibility) of the printed material, and thereby on the manufacturing process.<br/>After a presentation of some recent developments associated with the printing of dicyclopentadiene (DCPD)-based polymer and composite parts, we will discuss two challenges associated with the FP-based DIW method. The first one consists in the need to synchronize the printing and curing processes, as the propagation speed of the polymerization front must match that of the printer head. We demonstrate that, by taking advantage of the monotonic dependence of the front speed on the temperature of the gel, the polymerization front automatically adapts to changes in the print speed and in the thermal environment. A simple thermo-chemical model is introduced to capture the key features of this inherent closed-loop control of the 3D printing process.<br/>The second issue to be discussed is linked to the dimensional stability of the manufactured part with emphasis on the deformations of the uncured gel prior to the arrival of the polymerization front. To that effect, we develop a coupled thermo-chemo-structural model that incorporates the mechanically, thermally, and chemically driven sources of deformations and their contributions to the final shape of the printed part. The model, which incorporates the potentially large deformations of the gel, is based on an eigenstrain formulation that ‘freezes’ the deformations of the gel at the arrival of the polymerization front.